The use of hydraulic jars has been limited in the past primarily to "fishing" operations because of the hostile and rugged conditions encountered in drilling. For drilling use, the jar must be able to withstand long periods of service downhole and still remain operational when needed. Indeed a drilling jar may remain downhole for many days or even weeks before the drilling string is tripped to change the bit. The drilling process by its nature involves high torsional stresses and vibration, which a drilling jar must be rugged enough to withstand.
Prior art hydraulic drilling jars such as those shown in Love U.S. Pat. No. 3,566,981, Webb U.S. Pat. No. 4,109,736, and Sutliff U.S. Pat. No. 2,645,459, use a metering sleeve on the outside of a mandrel as a tripping mechanism. The sleeve is pulled through a reduced diameter section of the housing and into an enlarged diameter section thereof as hydraulic oil leaks through a small clearance space between the outer surface of the sleeve and the inner wall of such reduced diameter section. The slow leakage retards longitudinal movement of the mandrel to enable a stretch to be taken in the pipe string. When the metering sleeve reaches the enlarged diameter, a large bypass area is presented that allows free movement of the mandrel until the impact surfaces engage. During reverse movement of the mandrel, the lower end of the sleeve moves away from a seat on the mandrel to allow unrestricted movement to the cocked position. Although jars that include such metering sleeves have been widely used, the sliding surfaces are subject to wear which reduces their effectiveness as high pressure seals. The sleeves also are very expensive to manufacture to the close tolerances that are required. Other prior art jars have high pressure seals that are subject to contamination by drilling mud in the borehole, which reduces seal life. All of the prior art metering sleeve designs that applicant is aware of have internal chambers that are compensated to the outside by holes in the housing. Such holes create weak points that can cause fatigue failures when subjected to cyclical stresses during bent rotation.
The general object of the present invention is to provide a new and improved hydraulic jar that alleviates the above-mentioned problems with prior devices.
Another object of the present invention is to provide a new and improved jar of the type described having a metering valve that is easy and inexpensive to make and replace, and which eliminates the need to manufacture large and expensive parts that heretofore have had to be machined to close tolerances.
Yet another object of the present invention is to provide a new and improved jar having internal compensation so that no external holes are present which present weak points that can fail due to cyclical stresses in bent rotation.
Still another object of the present invention is to provide a new and improved jar that is uniquely arranged such that all high pressure seals are completely isolated from drilling mud to ensure longer life by eliminating contamination.
Another object of the present invention is to provide a new and improved drilling jar that includes a damping feature which prevents generation of a tensile shock wave in case the housing moves downward relative to the mandrel in a highly deviated borehole.